In ink jet printers, the printhead and recording medium are usually moved continuously relative to each other. During the movement, drops of ink are deflected to selected sites on the record medium along axes transverse to the path of motion. Because the drops are generated in succession from a nozzle, a straight line segment such as a character stroke, has an inclination in which the drops forming the segment do not lie on a line normal to the direction of motion.
The usual correction for the inclination has been to alter the direction of the deflecting force field for the drops an amount which will result in a non-inclined column of marks forming the line segment. One form of compensation is that of tilting the deflecting electrodes in electrostatic printing such as shown in U.S. Pat. Nos. 3,641,588 and 3,813,676. Another method has been to distort the force field by offsetting or skewing the deflecting electrodes such as disclosed in U.S. Pat. No. 3,895,386. Yet another technique has been the addition of a pair of compensating deflecting electrodes along the drop flight path which are normal to the principal deflection electrodes and impart a correcting amount of deflection to that produced by the principal electrodes, as shown in U.S. Pat. No. 3,938,163.
Frequently, it is desirable to record while the printhead is moving along both the forward and return strokes to increase printing throughput. The first two correction technques mentioned above require mechanical repositioning of the electrodes at the end of each line of print to provide a proper correction in the opposite direction. Such mechanical repositioning invites errors in the rapid adjustment necessary to maintain printing efficiency. When the printhead is moved in both directions with the same velocity, the positioning will be double the compensation amount, thus requiring the rapid movement of a relatively large mass. The last of the above-mentioned techniques has the disadvantage of increasing the drop flight path in order to acommodate the auxiliary electrodes. Such path extension necessitates greater flight time and the attendant adverse drop interaction and aerodynamic effects.
In U.S. Pat. No. 3,938,163, it is observed that drops may be scanned upward, known as forward rastering, or downward known commonly as reverse rastering, and that the angle of inclination of a column of drops will be dependent upon the direction of travel and the direction of rastering. U.S. Pat. application, Ser. No. 751,235 entitled "Bi-Directional Dot Matrix Printer" filed by L. V. Galetto et al. on Dec. 16, 1976 and assigned to the assignee of the present invention, has used forward and reverse rastering to avoid the adjustment of deflection electrodes when changing the direction of printing. Instead, the direction of raster is changed at the end of each printed line so that all characters have the same inclination or are oriented normal to the direction of travel of the printhead. Although this latter technique avoids adjustment of the deflection electrodes, it requires that the sequence of drop charging be reversed for each line.